Optical ultrasound receiver

ABSTRACT

An imaging guidewire can include one or more optical fibers communicating light along the guidewire. At or near its distal end, one or more blazed or other Fiber Bragg Gratings (FBGs) can direct light to a photoacoustic transducer material that provides ultrasonic imaging energy. Returned ultrasound can be sensed by an FBG sensor. A responsive signal can be optically communicated to the proximal end of the guidewire, and processed such as to develop a 2D or 3D image. In an example, the guidewire outer diameter can be small enough such that an intravascular catheter can be passed over the guidewire. To minimize the size of the guidewire, an ultrasound-to-acoustic transducer that is relatively insensitive to the polarization of the optical sensing signal can be used. The ultrasound-to-optical transducer can be manufactured so that it is relatively insensitive to the polarization of the optical sensing signal.

CLAIM OF PRIORITY

This patent application claims the benefit of priority of Michael J.Eberle et al. U.S. Provisional Patent Application Ser. No. 61/102,216,(Attorney Docket Number 1599.005PRV), which was filed on Oct. 2, 2009,and which is incorporated herein by reference in its entirety.

BACKGROUND

Bates et al. U.S. Pat. No. 7,245,789 is incorporated by reference hereinin its entirety, including its discussion of systems and methods forminimally-invasive optical-acoustic imaging. It discusses, among otherthings, an imaging guidewire that includes one or more optical fiberscommunicating light along the guidewire. At or near a distal end of theguidewire, light is directed to a photoacoustic transducer material thatprovides ultrasonic imaging energy. Returned ultrasound energy is sensedby an ultrasound-to-optical transducer. A responsive signal is opticallycommunicated to the proximal end of the guidewire, such that it can beprocessed to develop a 2D or 3D image.

OVERVIEW

Among other things, the present applicant has recognized thedesirability of reducing or minimizing the size of a system like thatdescribed in Bates. However, the present applicant has recognized thatreducing the size of the ultrasound-to-optical transducer can alsochange the mechanical acoustic resonance of the transducer at someultrasound frequencies and can also result in a generally smaller outputsignal. Additionally, the transducer response can become dependent onthe polarization of the optical signal since it is responsive toacoustic impedances in the direction of polarization. This can result inundesirable variations in the output signal conditional on thepolarization direction due to different resonances in different lateraldirections, for example. One approach to reduce or eliminate this effectwould be to control polarization of the optical signal. However, suchpolarization is difficult to control or predict in a cost-effectivemanner. Accordingly, the present applicant has recognized, among otherthings, that it can be advantageous to provide an ultrasound-to-opticaltransducer that is relatively insensitive to optical signalpolarization.

Example 1 can include an ultrasound-to-optical transducer, including anoptical fiber, an ultrasound-absorptive backing configured forsubstantially absorbing ultrasound energy that passes beyond the opticalfiber to reach the backing, and a Fiber Bragg Grating (FBG)interferometer configured for receiving the ultrasound energy andcapable of modulating an optical sensing signal, in response to theultrasound energy, substantially independent of a polarization angle ofthe optical sensing signal. In this example, the optical fiber caninclude an optical fiber diameter that is less than half of a wavelengthof target ultrasound energy to be received by the transducer.

In Example 2, the subject matter of Example 1 can optionally comprise anelongated intravascular imaging assembly, comprising a proximal externalportion comprising an optical signal interface and a distal portionsized and shaped and configured for internal intravascular acousticimaging. In this example, the imaging assembly can comprise an elongatedsupport, extending substantially from the proximal portion of theimaging assembly to the distal portion of the imaging assembly, thesupport configured to provide adequate length, rigidity, and flexibilityto permit intravascular introduction and steering of a distal portion ofthe imaging assembly to a location of interest, the optical fiber,extending longitudinally and affixed along the support substantiallyfrom the proximal portion of the imaging assembly substantially to thedistal portion of the imaging assembly, a transducer, located at or neara distal portion of the imaging assembly and configured as at least oneof the ultrasound-to-optical transducer or an optical-to-ultrasoundtransducer; and the backing, located in a backing region between thetransducer and the elongated support, the backing configured toattenuate ultrasound energy that reaches the backing region by at least90%.

In Example 3, the subject matter of any one of Examples 1 or 2 canoptionally be configured so that the assembly comprises a plurality ofthe optical fibers, extending longitudinally and affixed along thesupport, the fibers arranged circumferentially about a centrallongitudinal axis of the support, and a plurality of the transducers,wherein the transducer includes a photoacoustic material located at aperipheral portion of the optical fiber that is away from the backing.

In Example 4, the subject matter of any one of Examples 1-3 canoptionally be configured such that the optical fiber diameter is lessthan sixty micrometers.

In Example 5, the subject matter of any one of Examples 1-4 canoptionally be configured so that the backing comprises microballoonscontaining a gas.

In Example 6, the subject matter of any one of Examples 1-5 canoptionally be configured so that the microballoons comprise about 30% toabout 50% of the volume of the backing.

In Example 7, the subject matter of any one of Examples 1-6 canoptionally be configured so that a thickness of the backing is betweenabout 25 micrometers and about 100 micrometers.

In Example 8, the subject matter of any one of Examples 1-7 canoptionally comprise the transducer being configured such that apeak-to-peak amplitude of an oscillating output optical bias signalmodulated by the transducer varies by no more than one decibel when thepolarization angle of the optical sensing signal is varied by 90degrees.

In Example 9, the subject matter of any one of Examples 1-8 canoptionally be configured such that a total birefringence induced on anoptical signal with a wavelength of about 1550 nanometers transmittedwithin the optical fiber is less than (3×10⁻⁶).

Example 10 can include (or can be combined with the subject matter ofany one of Examples 1-9 to include) fabricating an ultrasound-to-opticaltransducer, the fabricating comprising providing an optical fiber thatincludes a diameter that is less than half of a wavelength of targetultrasound energy to be received by the transducer, creating a FiberBragg Grating (FBG) interferometer configured for receiving theultrasound energy and capable of modulating an optical sensing signalsubstantially independent of a polarization of the optical sensingsignal, associating an ultrasound-absorptive backing with the opticalfiber, and configuring the backing for substantially absorbingultrasound energy that passes beyond the optical fiber to reach thebacking.

In Example 11, the subject matter of any one of Examples 1-10 canoptionally comprise associating the backing comprising microballoonscontaining a gas with the fiber.

In Example 12, the subject matter of any one of Examples 1-11 canoptionally comprise providing the backing comprising the microballoonsthat comprise about 30% to about 50% of the volume of the backing.

In Example 13, the subject matter of any one of Examples 1-12 canoptionally comprise inserting the backing into a backing region betweena central support structure and the optical fiber, wherein both thesupport structure and the fiber extend longitudinally, the backingregion being bounded laterally by a tubular sheath.

In Example 14, the subject matter of any one of Examples 1-13 canoptionally comprise conforming the backing to the external surface of acentral support structure, wherein both the support structure and thefiber extend longitudinally.

In Example 15, the subject matter of any one of Examples 1-14 canoptionally further comprise conforming a tubular sheath to a surface ofthe backing that is away from the support structure.

Example 16 can include (or can be combined with the subject matter ofany one of Examples 1-15 to include) receiving ultrasound energy with anoptical fiber, the optical fiber including a diameter that is less thanhalf of a wavelength of the ultrasound energy, and using the ultrasoundenergy to modulate an optical sensing signal substantially independentof a polarization of the optical sensing signal.

In Example 17, the subject matter of any one of Examples 1-16 canoptionally comprise using the ultrasound energy to modulate the opticalsensing signal substantially independent of the polarization of theoptical sensing signal comprises generating a modulated output opticalsignal with a peak-to-peak amplitude that varies by no more than onedecibel when a polarization angle of the optical sensing signal isvaried by 90 degrees.

In Example 18, the subject matter of any one of Examples 1-17 canoptionally comprise using an ultrasound-absorptive backing tosubstantially attenuate ultrasound energy that passes beyond the opticalfiber to reach the backing.

In Example 19, the subject matter of any one of Examples 1-18 canoptionally comprise using the ultrasound-absorptive backing to attenuateultrasound energy that passes beyond the optical fiber to reach thebacking by at least 90%.

In Example 20, the subject matter of any one of Examples 16-19 canoptionally comprise forming an image of a region within a living bodyusing information from a modulated output optical signal.

These examples can be combined with each other in any permutation orcombination and or with other subject matter disclosed herein. Thisoverview is intended to provide an overview of subject matter of thepresent patent application. It is not intended to provide an exclusiveor exhaustive explanation of the invention. The detailed description isincluded to provide further information about the present patentapplication.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 is a cross-sectional side view illustrating generally, by way ofexample, but not by way of limitation, an example of an FBG strainsensor in an optical fiber.

FIG. 2 is a cross-sectional side view illustrating generally, by way ofexample, but not by way of limitation, an example of an FBG gratinginterferometer sensor.

FIG. 3 is a cross-sectional schematic diagram illustrating generally anexample of an acousto-optic transducer.

FIG. 4 is a cross-sectional schematic diagram illustrating generallyanother example of an FBG strain sensor in an optical fiber.

FIG. 5 is a schematic diagram that illustrates generally an example of aside view of a distal portion of a guidewire.

FIG. 6 is a schematic diagram that illustrates generally an example of across-sectional side view of a distal portion of a guidewire.

FIG. 7 is a schematic diagram that illustrates generally an example of across-sectional end view of a proximal portion of a guidewire.

FIG. 8 is a schematic diagram that illustrates generally an example of across-sectional end view of a distal portion of a guidewire.

FIG. 9 is a cross-sectional end view that illustrates generally anexample of a guidewire assembly at a location of a transducing window.

DETAILED DESCRIPTION 1. Examples of Fiber Bragg Grating Strain Sensors

FIG. 1 is a cross-sectional side view illustrating generally, by way ofexample, but not by way of limitation, an example of a strain-detectingor pressure-detecting acoustic-to-optical FBG sensor 100 in an opticalfiber 105. FBG sensor 100 senses acoustic energy received from a nearbyarea to be imaged, and transduces the received acoustic energy into anoptical signal within optical fiber 105. In the example of FIG. 1, FBGsensor 100 can include Bragg gratings 110A-B in an optical fiber core115, such as surrounded by an optical fiber cladding 120. Bragg gratings110A-B can be separated by a strain or pressure sensing region 125,which, in an example, can be about a millimeter in length. This examplecan sense strain or pressure such as by detecting a variation in lengthof the optical path between these gratings 110A-B.

A Fiber Bragg Grating can be conceptualized as a periodic change in theoptical refractive index of a portion of the optical fiber core 115.Light of specific wavelengths traveling down such a portion of core 115will be reflected; the period (distance) 130 of the periodic change inthe optical index determines the particular wavelengths of light thatwill be reflected. The degree of optical index change and the length 135of the grating determine the ratio of light reflected to thattransmitted through the grating 110A-B.

FIG. 2 is a cross-sectional side view illustrating generally, by way ofexample, but not by way of limitation, an operative example of aninterferometric FBG sensor 100. The example of FIG. 2 can include twoFBG mirrors 110A-B, which can be both partially reflective such as for aspecific range of wavelengths of light passing through fiber core 115.Generally, the reflectivity of each FBG will be substantially similar,but can differ for particular implementations. This interferometricarrangement of FBGs 110A-B can be capable of discerning the “opticaldistance” between FBGs 110A-B with extreme sensitivity. The “opticaldistance” can be a function of the effective refractive index of thematerial of fiber core 115 as well as the length 125 between FBGs110A-B. Thus, a change in the refractive index can induce a change inoptical path length, even though the physical distance 125 between FBGs110A-B has not substantially changed.

An interferometer, such as FBG sensor 100, can be conceptualized as adevice that measures the interference between light reflected from eachof the partially reflective FBGs 110A-B. When the optical path lengthbetween the FBG minors 110A-B is an exact integer multiple of thewavelength of the optical signal in the optical fiber core 115, then thelight that passes through the FBG sensor 100 will be a maximum and thelight reflected will be a minimum, so the optical signal issubstantially fully transmitted through the FBG sensor 100. Thisaddition or subtraction can be conceptualized as interference. Theoccurrence of full transmission or minimum reflection can be called a“null” and occurs at a precise wavelength of light for a given opticalpath length. Measuring the wavelength at which this null occurs canyield an indication of the length of the optical path between the twopartially reflective FBGs 110A-B. In such a manner, an interferometer,such as FBG sensor 100, can sense a small change in distance, such as achange in the optical distance 125 between FBGs 110A-B resulting fromreceived ultrasound or other received acoustic energy. This arrangementcan be thought of as a special case of the FBG Fabry-Perotinterferometer, sometimes more particularly described as an Etalon,because the distance 125 between the FBGs 110A-B is substantially fixed.

The sensitivity of an interferometer, such as FBG sensor 100, can dependin part on the steepness of the “skirt” of the null in the frequencyresponse. The steepness of the skirt can be increased by increasing thereflectivity of the FBGs 100A-B, which also increases the “finesse” ofthe interferometer. The present applicant has recognized, among otherthings, that increasing the finesse or steepness of the skirt of FBGsensor 100 can increase the sensitivity of the FBG sensor 100 to thereflected ultrasound signals within a particular wavelength range butcan decrease the dynamic range. As such, keeping the wavelength of theoptical sensing signal within the wavelength range can be advantageous.In an example, a closed-loop system can monitor a representativewavelength (e.g., the center wavelength of the skirt of the filteringFBG sensor 100) and can adjust the wavelength of an optical output laserto remain substantially close to the center of the skirt of the filtercharacteristic of the FBG sensor 100 as forces external to the opticalfiber 105, such as bending and stress, cause the center wavelength ofthe skirt of the filter characteristic of the FBG sensor 100 to shift.

In an example, such as illustrated in FIG. 2, the interferometric FBGsensor 100 can cause interference between that portion of the opticalbeam that is reflected off the first partially reflective FBG 110A withthat reflected from the second partially reflective FBG 110B. Thewavelength of light where an interferometric null will occur can be verysensitive to the “optical distance” between the two FBGs 110A-B. Theinterferometric FBG sensor 100 of FIG. 2 can provide another verypractical advantage. In this example, the two optical paths along thefiber core 115 are the same, except for the sensing region between FBGs110A-B. This shared optical path ensures that any optical changes in theshared portion of optical fiber 105 will have substantially no effectupon the interferometric signal; only the change in the sensing region125 between FBGs 110A-110B is sensed. However, the present applicant hasrecognized, among other things, that this sensing can be affected bybirefringence of the optical fiber within FBG sensor 100.

Optical birefringence is a measure of the difference in refractive indexof an optical medium for light of different polarizations. Thepolarization of light can be defined as the orientation of the electricvector of the electromagnetic light wave. Birefringence between two atleast partially reflective FBGs, such FBGs 110A-B, can cause differentbeams of light with different polarizations to effectively travelslightly different optical path lengths between the FBGs 110A-B.Therefore, a combination of birefringence between the at least partiallyreflective FBGs 110A-B and a shift in the polarization of the opticalsignal within the optical fiber core 115 can cause a shift in the exactwavelength of the null of the FBG sensor 100. When light is splitbetween different polarization states, the light will be reflected ortransmitted from different parts of the skirt of the null, which canlead to fading of the optically transduced signal. There are manypossible states of polarization, such as linear, circular, andelliptical, but the worst signal fading generally occurs when theoptical signal is split equally between linear polarization states thatare orthogonally aligned. With this in mind, the present applicant hasrecognized, among other things, that birefringence should be reduced orotherwise addressed, if possible.

The present applicant has also recognized that there can be two mainsources of optical birefringence within the FBG sensor 100. The firstsource is the intrinsic birefringence of the optical fiber 105. Theintrinsic birefringence is determined mostly during the manufacturing ofthe optical fiber 105 and is generally a function of the level ofgeometric symmetry, the uniformity of dopant distribution, and the levelof stress in the fiber core 115 during the drawing of the fiber core115.

The second source of optical birefringence in the FBG sensor 100 is theprocess of writing the FBGs 110A and 110B. The present applicant hasalso recognized that the birefringence induced by the writing of FBGs110A and 110B can be reduced such as by controlling one or more aspectsof the writing process, such as the polarization of the writing laser,the laser pulse energy, the writing exposure time, the amount ofhydrogen or deuterium in the fiber during writing, or any combinationthereof.

2. Examples of FBG Acoustic-to-Optical Transducers

In an example, an FBG sensor 100 senses pressure or strain such asgenerated by ultrasound or other acoustic energy received from a nearbyimaging region to be visualized and, in response, modulates an opticalsensing signal in an optical fiber. Increasing the sensitivity of theFBG sensor 100 can provide improved imaging. A first example ofincreasing sensitivity is to increase the amount of strain induced inthe FBG sensor 100 for a given dynamic pressure provided by the acousticenergy. A second example is to increase the modulation of the opticalsignal for a given change in strain of the FBG sensor 100. Anycombination of the techniques of these first and second examples canalso be used.

One technique of increasing the strain induced in the FBG sensor 100 isto configure the physical attributes of the FBG sensor 100 such as toincrease the degree of strain for a given externally-applied acousticfield. In an example, the FBG sensor 100 can be shaped so as to increasethe strain for a given applied acoustic pressure field. FIG. 3 is across-sectional schematic diagram illustrating such an example in whichthe FBG sensor 100 is shaped such that it mechanically resonates at ornear the frequency of the acoustic energy received from the nearbyimaging region, thereby resulting in increased strain. In the example ofFIG. 3, all or a portion of the strain sensing region between FBGs110A-B is selected to provide a thickness 300 that promotes suchmechanical resonance of the received acoustic energy, thereby increasingthe resulting strain sensed by FBG sensor 100. In an example, such asillustrated in FIG. 3, this can be accomplished by grinding or otherwiseremoving a portion of fiber cladding 120, such that the remainingthickness of fiber core 115 or fiber cladding 120 between opposingplanar (or other) surfaces is selected to mechanically resonate at ornear the frequency of the acoustic energy received from the nearbyimaging region.

In an example, mechanical resonance can be obtained by making thethickness 300 of the strain sensing region substantially the samethickness as ½ the acoustic wavelength (or an odd integer multiplethereof) in the material(s) of FBG sensor 100 at the acoustic centerfrequency of the desired acoustic frequency band received from theimaging region. In other examples, such as for other materials, thethickness 300 can be selected to match a different proportion of theacoustic wavelength that obtains the desired mechanical resonance forthat material. Calculations indicate that obtaining such mechanicalresonance can increase the strain sensitivity by a factor of 2 or moreover that of a sensor that is not constructed to obtain such mechanicalresonance.

In a third example, a coating 305 can be applied to the FBG sensor 100such as to increase the acoustic pressure as seen by the FBG sensor 100over a band of acoustic frequencies, such as for improving itssensitivity over that band. The difference between the mechanicalcharacteristics of water (or tissue and/or blood, which is mostlycomprised of water) and glass material of the optical fiber 105 carryingthe FBG sensor 100 is typically so significant that only a small amountof acoustic energy “enters” the FBG sensor 100 and thereby causesstrain; the remaining energy is reflected back into the biological orother material being imaged. For a particular range of acousticfrequencies, one or more coatings 305 of specific thickness 310 ormechanical properties (e.g., the particular mechanical impedance) of thecoating material can be used to dramatically reduce such attenuation dueto the different mechanical characteristics. An example can use quarterwave matching, providing a coating 305 of a thickness 310 that isapproximately equal to one quarter of the acoustic signal wavelengthreceived from the region being imaged. Using such matching, thesensitivity of the FBG sensor 100, over a given band of acousticfrequencies of interest, is expected to increase by about a factor of 2.

However, for a mechanically resonant transducer, reducing the thicknessof fiber cladding 120, and therefore reducing the total size of thetransducer, is limited by the wavelength of the acoustic energy to bereceived. In a fourth example, the thickness 300 of the strain sensingregion is substantially less than half of the wavelength of the targetacoustic energy. FIG. 4 is a cross-sectional schematic diagramillustrating such an example. In this example, the thickness 300 of thestrain sensing region can be less than 60 micrometers. As the acousticwidth of the transducer is reduced, the cross-sectional area of thetransducer is reduced, and the total strain due to acoustic pressure onthe transducer is reduced. Therefore, a smaller thickness 300 of thestrain sensing region results in a smaller received acoustic signal,leading to reduced receive sensitivity of the FBG sensor 100. Thisreduced receive sensitivity of the FBG sensor 100 can be improved byplacing a backing 440 between the strain sensing region of the opticalfiber 105 and a core wire 450 that can be configured to provide support,rigidity, and flexibility for the assembly. The backing 440 can includea high acoustic impedance material such as a metal or ceramic. In anexample, the backing 440 can be separated from the optical fiber such asby an acoustically thin (e.g., less than a quarter wavelength thick)tubular sheath 460. The coating 305 can also be configured, in anexample, for acoustic impedance matching.

In an example, the backing 440 can be configured to reflect acousticenergy back to the strain sensing region, such as in a resonant manner,to increase the total amount of acoustic energy received by the strainsensing region and thereby increase the strain on the region. In thisexample, however, the receive sensitivity of FBG sensor 100 may dependon the polarization of the optical sensing signal because the backing440 does not enhance the acoustic signal in all directions. The receivesensitivity generally is highest when the optical sensing signal ispolarized parallel to an axis extending through the backing 440 and thefiber core 115 or an axis extending through the core wire 450 and thefiber core 115. The receive sensitivity is generally lowest when theangle of the polarization of the optical sensing signal is parallel tothe outline of the interface between the backing 440 and the sheath 460,because the backing does not enhance the signal for this polarizationorientation. The variable sensitivity in this example results at leastpartially from acoustic reflections from the backing 440 and from thecore wire 450. The present applicant has recognized, among other things,that sensitivity to polarization of the optical sensing signal should bereduced or eliminated, if possible, in an example.

In an example, the backing 440 can be configured to absorb the acousticenergy that penetrates completely through the optical fiber to reach thebacking 440. In this example, the backing 440 can be configured both tonot reflect any acoustic energy back to FBG sensor 100 and also toinhibit or prevent the core wire 450 from reflecting any acoustic energyback to FBG sensor 100. The sheath 460 in this example can be configuredto not reflect any substantial amount of acoustic energy. In anillustrative example, the sheath 460 can comprise a layer of UV-curablepolyester such as with a thickness of about 10 micrometers or less. Thebacking 440 in this illustrative example can comprise microballoonsfilled with gas or microballoons filled with gas mixed into a polymermatrix. In an example of a device configured for imaging within acoronary vasculature, the room for the backing 440 is very limited, andtherefore it can be advantageous if the backing 440 is relatively highlyacoustically absorbent with a relatively small thickness. In an examplein which acoustic energy that penetrates completely through the opticalfiber to reach the backing 440 is substantially absorbed by a relativelysmall thickness of the backing 440, microballoons can comprise 30% to50% of the volume of the backing 440. With such a poymer-microballoonmixture, the backing 440 can have a thickness of as little as 50micrometers and can be configured to be capable of attenuating acousticenergy with a frequency of 20 megahertz by 17 decibels to 23 decibelsper millimeter of thickness of the backing 440. In an example, thesensitivity of FBG sensor 100 is relatively independent of thepolarization of the output optical beam within FBG sensor 100. In anexample, a peak-to-peak amplitude of the oscillating output opticalsensing signal varies by no more than one decibel as the polarization ofthe output optical sensing signal is rotated 90 degrees. In an example,the backing 440 attenuates ultrasound energy by at least 90%.

The above examples can be combined with each other or can include moreor fewer elements than are recited in the examples and can stillfunction as described in the examples. For example, anultrasound-to-optical transducer that is relatively insensitive to thepolarization of the optical sensing signal can include an optical fiber,a backing configured to absorb ultrasound energy that goes through theoptical fiber to reach the backing, and an FBG interferometer configuredto modulate an optical sensing signal in response to the ultrasoundenergy. The optical fiber can include a thickness of the optical fiberthat is less than half the wavelength of the ultrasound energy that issensed by the transducer.

3. Examples of Guidewire Design

FIG. 5 is a schematic diagram that illustrates generally an example of aside view of a distal portion 500 of an imaging guidewire 505 or otherelongated catheter (in an example, the guidewire 505 is sized and shapedand is of flexibility and rigidity such that it is capable of being usedfor introducing and/or guiding a catheter or other medical instrument,e.g., over the guidewire 505 within a living body). In this example, thedistal portion 500 of the imaging guidewire 505 includes one or moreimaging windows 510A, 510B, . . . , 510N located slightly orconsiderably proximal to a distal tip 515 of the guidewire 505. Eachimaging window 510 includes one or more acoustic-to-optical FBG sensors100. In an example, the different imaging windows 510A, 510B, . . . ,510N are designed for different optical wavelengths, such thatparticular individual windows can be addressed by changing the opticalwavelength being communicated through fiber core 115.

FIG. 6 is a schematic diagram that illustrates generally an example of across-sectional side view of a distal portion 600 of a guidewire 605. Inthis example, the guidewire 605 can include a solid metal or other corewire 450 that can taper down in diameter (e.g., from an outer diameterof about 0.011 inches) at a suitable distance 615 (e.g., about 50 cm)from the distal tip 620, to which the tapered-down distal end of corewire 450 can be attached. In this example, optical fibers 625 can bedistributed around the outer circumference of the guidewire core 450,and can be attached to the distal tip 620. In this example, the opticalfibers 625 can be at least partially embedded in a binder material(e.g., UV curable acrylate polymer) that bonds the optical fibers 625 tothe guidewire core wire 450 or the distal tip 620. The binder materialmay also contribute to the torsion response of the resulting guidewireassembly 605. In an example, the optical fibers 625 and binder materialcan be overcoated with a polymer or other coating 630, such as forproviding abrasion resistance, optical fiber protection, or frictioncontrol, or a combination thereof. At least one metallic or otherbulkhead 640 can be provided along the tapered portion of the core wire450. In an example, the optical fibers 625 and binder 635 can beattached to a proximal side of the bulkhead 640 such as near itscircumferential perimeter. A distal side of the bulkhead 640 can beattached, such as near its circumferential perimeter, to a coil winding610 that can extend further, in the distal direction, to a ball or otherdistal tip 620 of the guidewire 605. In this example, the compositestructure of the distal region 600 of the guidewire 605 can provide,among other things, flexibility and rotational stiffness, such as forallowing the guidewire 605 to be maneuvered to an imaging region ofinterest such as within a vascular or any other lumen.

FIG. 7 is a schematic diagram that illustrates generally an example of across-sectional end view of a proximal portion 700 of guidewire 605,which can include core wire 450, optical fibers 625, binder material635, and outer coating 630. In this example, but not by way oflimitation, the diameter of the core 450 can be about 11/1000 inch, thediameter of the optical fibers 625 can be about (1.25)/1000 inch, andthe optional outer coating 1030 can be about (0.25)/1000 inch thick.

FIG. 8 is a schematic diagram that illustrates generally an example of across-sectional end view of the distal portion 600 of the guidewire 605,e.g., adjacent to the distal tip 620. In this example, but not by way oflimitation, the diameter of core wire 450 has tapered down to about1.5/1000 inch, circumferentially surrounded by a void 800 of about thesame outer diameter (e.g., about 11/1000 inch) as the core wire 450 nearthe proximal end 700 of the guidewire 605. In this example, the opticalfibers 625 can be circumferentially disposed in the binder material 635around the void 800. Binder material 635 can provide structural support.Optical fibers 625 can be optionally overlaid with the outer coating630.

4. Examples of Acoustic Transducer Construction

In an example, before one or more acoustic transducers are fabricated,the guidewire 605 can be assembled. FIG. 9 is a cross-sectional end viewillustrating an example of a structure of such a guidewire assembly suchas at a location of a transducing window 510. An example of suchassembling can include placing the tubular sheath 460 on the core wire450 such as at the locations selected for transducing, inserting thebacking 440 into a gap between the core wire 450 and the tubular sheath460, and binding the optical fibers 625 to the sheath 460 and the distaltip 620 or bulkhead 640. The coating 305, which, in an example, caninclude the outer coating 630, can then optionally be layered over theoptical fibers 625. In another example of such assembling, the backing440 can be formed to the surface of the core wire 450 such as at thelocations selected for transducing, and the tubular sheath 460 can beformed to the exposed surface of the backing, such as by heat-shrinking,for example. In an example, the optical fibers 625 can each have adiameter of less than half of the wavelength of the acoustic energy thatthe acoustic transducers are designed to sense, although the diameter ofeach of the optical fibers 625 can be larger than this in otherexamples.

After the guidewire 605 has been assembled, the FBGs can be added to oneor more portions of the optical fibers 625, such as within thetransducer windows 510. In an example, an FBG can be created using anoptical process in which a portion of the optical fiber 625 is exposedto a carefully controlled pattern of ultraviolet (UV) radiation thatdefines the Bragg grating. Then, a photoacoustic material or otherdesired overlayer can be deposited or otherwise added in the transducerwindows 510 such as over the Bragg grating. Thus, in this example, theFBGs can be advantageously constructed after the optical fibers 625 havebeen mechanically assembled into the guidewire assembly 605.

An FBG writing laser can be used to expose the desired portion of theoptical fiber 625 to a carefully controlled pattern of UV radiation todefine the Bragg grating. The FBG writing laser can be operated so as toreduce the amount of birefringence caused by the FBGs. This will reducethe dependence of the FBG sensor 100 on the polarization of the opticalsensing signal that is modulated by the received acoustic energy. In anillustrative example, at least one of hydrogen or deuterium can first beoptionally infused into the optical fiber core 115. Conditions fordiffusion of the gas into the fiber can be 150 atm pressure at roomtemperature for 1 to 10 days. Then, the optical fiber 625 can be exposedto the writing laser, such as for a time period of between about 30seconds and about 10 minutes. In this example, the writing laser canhave a pulse energy of between about 0.1 millijoules (mJ) and about 10millijoules and can have a polarization angle that is substantiallyparallel to the longitudinal axis of the optical fiber core 115.

In an illustrative example, such as in which hydrogen is not infusedinto the optical fiber core 115, the optical fiber 625 can be exposed tothe writing laser such as for a time period of between about 30 secondsand about 10 minutes. In this example, the writing laser can have apulse energy of between about 0.1 millijoules and about 10 millijoulesand a polarization angle that is substantially perpendicular to thelongitudinal axis of the optical fiber core 115. In either this exampleor the above example in which hydrogen or deuterium is first optionallyinfused into the optical fiber core 115, desired portions of all layersor coverings over the fiber cladding 120 can optionally be removedbefore the optical fiber 625 is exposed to the writing laser.

In an example, the writing conditions can be controlled so that the FBGsensor 100 is relatively insensitive to the polarization of the outputoptical signal. This can include, for example, reducing the speed ofwriting by lowering the intensity of the UV lamp in order to reduceheating biases. In an example, the shift in the center wavelength of theskirt of the FBG sensor 100 as the polarization of the output opticalsensing signal is rotated 90 degrees is less that half of the Full WidthHalf Maximum of the skirt of the FBG sensor 100. In another example, atotal birefringence induced on an optical signal with a wavelength ofabout 1550 nanometers transmitted within the optical fiber 625 is lessthan (3×10⁻⁶).

Additional Notes

In this document, the term “minimally-invasive” refers to techniquesthat are less invasive than conventional surgery; the term“minimally-invasive” is not intended to be restricted to theleast-invasive technique possible.

Although certain of the above examples have been described with respectto intravascular imaging (e.g., for viewing or identifying vulnerableplaque), the present systems, devices, and methods are also applicableto imaging any other body part. For example, the guidewire or otherelongated body as discussed above can be inserted into a biopsy needle,laparoscopic device, or any other lumen or cavity such as for performingimaging. Moreover, such imaging need not involve insertion of anelongate body into a lumen, for example, an imaging apparatus can bewrapped around a portion of a region to be imaged.

In an example, the present systems, devices, and methods can be used toprocess the Doppler shift in acoustic frequency to image or measureblood flow. The operation can be similar to that described above,however, this would increase the length of the transmitted acousticsignal, and can use Doppler signal processing in the image processingportion of the control electronics. The transmitted acoustic signal canbe lengthened, in an example, such as by repeatedly pulsing the transmitoptical energy at the same rate as the desired acoustic frequency.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Such examples can include elements in addition tothose shown and described. However, the present inventors alsocontemplate examples in which only those elements shown and describedare provided.

All publications, patents, and patent documents referred to in thisdocument are incorporated by reference herein in their entirety, asthough individually incorporated by reference. In the event ofinconsistent usages between this document and those documents soincorporated by reference, the usage in the incorporated reference(s)should be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the respectiveterms “comprising” and “wherein.” Also, in the following claims, theterms “including” and “comprising” are open-ended, that is, a system,device, article, or process that includes elements in addition to thoselisted after such a term in a claim are still deemed to fall within thescope of that claim. Moreover, in the following claims, the terms“first,” “second,” and “third,” etc. are used merely as labels, and arenot intended to impose numerical requirements on their objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, the code may be tangibly stored on one ormore volatile or non-volatile computer-readable media during executionor at other times. These computer-readable media may include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. An apparatus comprising: an ultrasound-to-optical transducer,comprising: an optical fiber, including an optical fiber diameter thatis less than half of a wavelength of target ultrasound energy to bereceived by the transducer; an ultrasound-absorptive backing, configuredfor substantially absorbing ultrasound energy that passes beyond theoptical fiber to reach the backing; and a Fiber Bragg Grating (FBG)interferometer configured for receiving the ultrasound energy andcapable of modulating an optical sensing signal, in response to theultrasound energy, substantially independent of a polarization angle ofthe optical sensing signal.
 2. The apparatus of claim 1, wherein theoptical fiber diameter is less than sixty micrometers.
 3. The apparatusof claim 1, wherein the backing comprises microballoons containing agas.
 4. The apparatus of claim 3, wherein the microballoons compriseabout 30% to about 50% of a volume of the backing.
 5. The apparatus ofclaim 4, wherein a thickness of the backing is between about 25micrometers and about 100 micrometers.
 6. The apparatus of claim 1,wherein the transducer is configured such that a peak-to-peak amplitudeof an oscillating output optical bias signal modulated by the transducervaries by no more than one decibel when the polarization angle of theoptical sensing signal is varied by 90 degrees.
 7. The apparatus ofclaim 1, wherein a total birefringence induced on an optical signal witha wavelength of about 1550 nanometers transmitted within the opticalfiber is less than (3×10⁻⁶).
 8. The apparatus of claim 1 comprising: anelongated intravascular imaging assembly, comprising an externalproximal portion comprising an optical signal interface and a distalportion sized and shaped and configured for internal intravascularacoustic imaging, the imaging assembly comprising: an elongated support,extending substantially from the proximal portion of the imagingassembly to the distal portion of the imaging assembly, the supportconfigured to provide adequate length, rigidity, and flexibility topermit intravascular introduction and steering of a distal portion ofthe imaging assembly to a location of interest; the optical fiber,extending longitudinally and affixed along the support substantiallyfrom the proximal portion of the imaging assembly substantially to thedistal portion of the imaging assembly; a transducer, located at or neara distal portion of the imaging assembly and configured as at least oneof the ultrasound-to-optical transducer or an optical-to-ultrasoundtransducer; and the backing, located in a backing region between thetransducer and the elongated support, the backing configured toattenuate ultrasound energy that reaches the backing region by at least90%.
 9. The apparatus of claim 8, wherein the assembly comprises: aplurality of the optical fibers, extending longitudinally and affixedalong the support, the fibers arranged circumferentially about a centrallongitudinal axis of the support; and a plurality of the transducers,wherein the transducer includes a photoacoustic material located at aperipheral portion of the optical fiber that is away from the backing.10. A method comprising: fabricating an ultrasound-to-opticaltransducer, the fabricating comprising: providing an optical fiber thatincludes a diameter that is less than half of a wavelength of targetultrasound energy to be received by the transducer; creating a FiberBragg Grating (FBG) interferometer configured for receiving theultrasound energy and capable of modulating an optical sensing signalsubstantially independent of a polarization of the optical sensingsignal; associating an ultrasound-absorptive backing with the opticalfiber; and configuring the backing for substantially absorbingultrasound energy that passes beyond the optical fiber to reach thebacking.
 11. The method of claim 10, wherein associating the backingwith the fiber comprises associating the backing comprisingmicroballoons containing a gas with the fiber.
 12. The method of claim11, wherein associating the backing comprising microballoons with thefiber comprises providing the backing comprising the microballoons thatcomprise about 30% to about 50% of the volume of the backing.
 13. Themethod of claim 12, wherein associating the backing comprisingmicroballoons comprises inserting the backing into a backing regionbetween a central support structure and the optical fiber, wherein boththe support structure and the fiber extend longitudinally, the backingregion being bounded laterally by a tubular sheath.
 14. The method ofclaim 12, wherein providing the backing comprises conforming the backingto the external surface of a central support structure, wherein both thesupport structure and the fiber extend longitudinally.
 15. The method ofclaim 14, wherein providing the backing further comprises conforming atubular sheath to a surface of the backing that is away from the supportstructure.
 16. A method comprising: receiving ultrasound energy with anoptical fiber, the optical fiber including a diameter that is less thanhalf of a wavelength of the ultrasound energy; and using the ultrasoundenergy to modulate an optical sensing signal, the modulation beingsubstantially independent of a polarization of the optical sensingsignal.
 17. The method of claim 16, wherein using the ultrasound energyto modulate the optical sensing signal, the modulation beingsubstantially independent of the polarization of the optical sensingsignal, comprises generating a modulated output optical signal with apeak-to-peak amplitude that varies by no more than one decibel when apolarization angle of the optical sensing signal is varied by 90degrees.
 18. The method of claim 16, comprising using anultrasound-absorptive backing to substantially attenuate ultrasoundenergy that passes beyond the optical fiber to reach the backing. 19.The method of claim 18, comprising using the ultrasound-absorptivebacking to attenuate ultrasound energy that passes beyond the opticalfiber to reach the backing by at least 90%.
 20. The method of claim 16,comprising forming an image of a region within a living body usinginformation from a modulated output optical signal.